1. Field of the Invention
The present invention relates to a transmission signal processing device for video signals and multi-channel audio signals, such as an AV amplifier, and also to a video and audio reproducing system in which a video device such as a display is added to the transmission signal processing device.
2. Description of the Related Art
A video and audio reproducing system including a display with a built-in speaker may be used to reproduce a content item containing a video image and a multi-channel audio, such as a movie. FIG. 8 shows a conventional video and audio reproducing system, including a display device 90, an AV amplifier 70, a front left speaker 3, a front right speaker 4, a surround left speaker 5, a surround right speaker 6, and a subwoofer 7. A video signal V and a volume-controlled, amplified front center audio signal C are supplied from the AV amplifier 70 to the display device 90 via an output terminal 72 and an output terminal 78, respectively. The display device 90 includes a display 91 connected to a video input terminal 92, and a left built-in speaker 93 and a right built-in speaker 94 connected to an audio input terminal 98 for reproducing a stereophonic sound. Therefore, in the conventional video and audio reproducing system, the front center audio signal (center signal) included in the multi-channel audio can be reproduced through the built-in speakers. In other words, the conventional video and audio reproducing system uses the built-in speakers of the display device as a substitute for a center speaker (not shown).
There are conventional television sets provided with a switch for switching between input signal terminals or speakers so as to allow the built-in display speaker to be used as a substitute for a center speaker for reproducing a front center audio signal (center signal) of a multi-channel audio. When reproducing a multi-channel audio, the built-in display speaker receives an amplified center signal from an external device such as an amplifier (Patent Document 1: Japanese Laid-Open Patent Publication No. 5-244685, Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-69379).
There are conventional systems where video signals and audio signals are exchanged between the display and the AV amplifier by a transmission signal format such as HDMI (High-Definition Multimedia Interface) or IEEE1394 (i-Link) in which video signals and multi-channel audio signals are multiplexed together. Such a transmission signal format requires only one connection cable for connecting together the display and the AV amplifier of the video and audio reproducing system, via which video signals and multi-channel audio signals can be exchanged.
However, if the display and the AV amplifier are connected to each other by a transmission signal format such as HDMI or IEEE1394 in which video signals and multi-channel audio signals are multiplexed together, in a video and audio reproducing system as shown in FIG. 8 where a built-in display speaker is used as a substitute for a center speaker, it will be difficult to control the volume of the front center audio signal, rendering the system practically unusable. The overall volume level of the video and audio reproducing system, i.e., the master volume, is controlled by the AV amplifier, which decodes and amplifies the multi-channel audio signal and outputs the obtained signal to the speaker. However, the multi-channel audio signal multiplexed in a format such as HDMI or IEEE1394 is undecoded data whose volume is not controlled by the master volume, whereby it is not possible to control the volume of the front center audio signal.